1. Field of the Invention
This invention relates to special effects produced on video images.
2. Description of the Prior Art
Special effects of certain types consist in combining a first and a second image obtained from different sources in order to generate an image in which one portion is supplied by the first image and in which another portion is supplied by the second image. In order to produce this effect, it is the customary practice to employ a device known as an electronic shape generator. When associated with a device designated as a variable-gain switcher, this generator serves to combine two video signals in order to obtain an image in which certain portions are provided by one of the video signals whilst the other portions are provided by the other video signal. For example, a shape generator associated with a variable-gain switcher permits the formation of an image having a background which is supplied by a first video signal and comprises an inset with blurred edges supplied by a second video signal. It is desirable to have the possibility of producing very different and variable shapes according to the requirements of users.
In order to generate the signals for controlling the switcher, conventional electronic shape generators comprise sawtooth-signal generators and parabolic signal generators. These signal generators can be either analog or digital generators. The analog generators have deficient time stability by reason of temperature variations, tolerances of components, aging, and so on. Digital generators are not subject to the problem of stability of adjustments and are easier to adjust. However, the shapes obtained are simple figures or patterns and it is particularly true to state that only a limited number of geometrical transformations can be carried out on these shapes. It is known, for example, to carry out a rotation of a square but not of a lozenge or of a star. It is also known to form blurred edges or colored borders, to perform a translational displacement or a multiplication of patterns but without any possibility of effecting a rotation at the same time.
The method in accordance with the invention makes it possible not only to generate a greater number of shapes but also permits transformation of these shapes by means of a translational displacement, an affine transformation, a movement of rotation, and an effect of perspective, which can be combined. This method also permits the formation of shapes by "modulating" a simple figure in a direction at right angles to its layout plane, independently of the directions of scanning of the image, which is not possible in the case of known devices.